Some current dust-proof and gas-proof masks contain filter units that are made from a nonwoven synthetic fiber filter material such as polypropylene, which is also used in air filters. Electrostatic filters have been proposed to reduce breathing resistance and mechanical filters which use a fiberglass fiber as the base material have also been proposed.
Unfortunately, there are many problems with these filters. For example, in the case of electrostatic filters, microfibers adhere to the filter material and collect in the folds. Oil mist reduces the electrostatic properties and lowers the dust collection efficiency of the filter. In the case of mechanical filters whose base material is fiberglass, the high dust collection efficiency produces increased resistance (inhalation resistance in the case of masks). It would be desirable therefore to have a filter unit for air filters, dust-proof masks and gas-proof masks with low pressure drop, (i.e., low inhalation resistance) and superior dust collection efficiency. The inhalation resistance and filter area have to be such that the filter unit can be placed inside a restricted area in front of an air filter unit or dust-proof mask. Each filter also has to be designed to collect as much dust as possible. A pleated filter with an expanded filter area has been disclosed (Japanese Unexamined Patent Application Publication [Kokai] No. 8-168629) which reduces inhalation resistance and increases the amount of dust collected.
In order to filter out dust particles the size of a single micron, the inhalation resistance of the filter has to be configured to accommodate fibers the size of a single micron. When a filter consisting of fibers the size of a single micron is folded into pleats, the filter has to be reinforced so as to be strong enough to withstand the air pressure due to inhalation resistance and also to resist mechanical vibration. The mechanical strength of the material used in the folded filter is weak, but this problem can be solved by increasing the thickness of the filter using a reinforcing material. This has to be done while maximizing the dust collection efficiency, minimizing inhalation resistance, and allowing the filter unit to be easily manufactured.